This thesis reports on experimental and numerical investigations of ion acceleration and the underlying mechanisms of energy transfer in the interaction of intense laser pulses with ultra-thin foils undergoing relativistic induced transparency. The optimisation and optical control of the ion beam properties including the beam flux, maximum energy and energy spread is important for the development of applications of laser-driven ion beams. Multiple laser-ion acceleration mechanisms, driven by sheath fields, radiation pressure and transparency enhancement occur in intense laser pulse interactions with an ultra-thin foil. This is experimentally and numerically demonstrated in the work presented in this thesis. Results from an experimental investigation of ion acceleration from ultra-thin (nanometer-thick) foils using the Vulcan petawatt laser facility are presented. Spatially separating the multiple beam components arising from the differing acceleration mechanisms enables the underlyi ng physics of the individual mechanisms to be investigated. In the case of foils undergoing relativistic induced transparency, it is shown that an extended channel and resulting jet is formed in the expanding plasma at the rear of the target, resulting in higher laser energy absorption into electrons and enhanced ion acceleration in a localised region. This results from volumetric heating of electrons by the laser pulse propagating within the channel. The measured maximum energy of the protons in the enhanced region of the jet is found to be highly sensitive to the laser pulse contrast and rising edge intensity profile of the laser. It is shown, using a controlled pre-expansion of the target, that an increase in the maximum proton energy by a factor two is achievable. Numerical investigations of the interaction, using particle-in-cell (PIC) simulations, show that an idealised sharp rising edge Gaussian laser intensity profile produces the highest proton energy, though this condition could not be achieved experimentally. The simulations show that controlled pre-expansion of the target, by variation of the rising edge intensity profile, enables better conditions for channel formation and energy coupling to electrons and thus protons. A detailed numerical (PIC) investigation of the mechanisms of laser energy transfer to electrons and ions in thin foils undergoing relativistically induced transparency is also presented. The role of streaming instabilities in the transfer of energy between particle species is investigated. It is found that in addition to the relativistic Buneman instability, which arises from streaming of the volumetrically heated relativistic electrons with the background ions during transparency, ionion streaming in the expanding plasma also plays a role in enhancing the final ion energy. Enhancement of proton maximum energies via ion-ion streaming from shock-accelerated aluminium ions is observed in 1D PIC simulations and the energy exchange is demonstrated to be sensitive to the plasma density. Energy transfer between co-directional ion species is also observed in higher dimension 2D simulations. The simulations show that the greatest enhancement in proton energy is due to streaming of electrons in the region of the plasma jet formed in the expanding plasma.